Pulley Backend to run Lua scripts

See the lua directory. All tests except LDAP have succeeded.

Note that the name of the Lua script is currently hard-coded, so this is not "feature complete" yet. Several paths lay open, including a _script="..." config parameter with optional extras to come later, like _mntns="/path/to/fd" to allow a control program to enter the filesystem of a managed container.

Synchronised with preliminary and incomplete work on the lifecycle backend.